An independent, peer reviewed journal for policy makers and practitioners in agriculture and related industries, assessing the interactions between population growth, resources, the environment and climate change.

Climate change, crop plant diseases and future food production

Over the next 100 years, with increasing levels of CO2 and O3, global temperatures are predicted to rise by 2°to 4 °C.

Extreme weather events such as drought, heat waves and erratic rainfall patterns are also likely to become more common. Changes in climate will affect different parts of world agriculture in different ways. Rising temperatures in tropical regions will be detrimental to crop production. In marginal, semi-arid areas of sub-Saharan Africa and South Asia, crop production may no longer be possible due to heat and drought.

However, increased warming in temperate areas may increase crop production through longer growing seasons. Concurrently, some changes in climate are likely to affect the diseases which cause crop losses.

In recent years, there has been significant growth in the body of literature on how climate change is likely to affect plant diseases1.

The majority of these papers predict that plant diseases are likely to become more severe, epidemics will be more frequent and some pathogens will spread to new areas. This literature warns of the challenges for agricultural research which must respond to these predicted changes.

It also highlights the uncertainties as to where, when and if plant diseases will become more severe, causing greater crop losses and threatening future food production, particularly in the latter half of this century2. Most papers conclude that there is a need for much more research and increased funding.

Decades of research have generated considerable knowledge and greater understanding of the seasonal effects of temperature, rainfall and humidity on diseases affecting major food crops. However, long-term data sets on plant disease development under changing environmental conditions are rare. It is also clear that plant diseases respond to different climatic variables in different ways3.

The outcome for any given host–pathogen interaction under changing climate is not readily predictable. Furthermore, the lack of models with multiple climate change parameters adds another layer of uncertainty, as there could be significant interactions between these variables.

The many factors involved in determining plant health under a changing climate, their direct and indirect effects, interactions and feedback loops raise the question of whether a predictive understanding of these complex systems is achievable4.

It is therefore important to make clear the inherent uncertainty in models of plant disease development under climate change1.

Two recent studies on wheat rusts illustrate this uncertainty. In one study in Europe, wheat leaf rust (Puccinia triticina) model scenarios using temperature and leaf wetness predicted disease onset would be one month earlier, due to rising temperatures during the latter part of this century5. This eﬀect was partly counter-balanced during the spring by lower leaf wetness frequency.

However, another study6 of several wheat leaf rusts noted that temperature rises had a beneficial effect on the pathogens' survival depending on availability of humidity and leaf wetness. On the other hand, increased levels of O3 had a negative effect while the effect of increased CO2 varied among susceptible cultivars. Without including temperature, leaf wetness, CO2 and O3 as parameters in the same model, it would therefore be difficult to predict whether wheat leaf rust would be more or less severe.

Well-developed models are available for major food crops. Models for plant diseases, however, are restricted to a few major pathogens7.

It is therefore not surprising that most modelling studies of plant diseases under changing climate are focused on major diseases of wheat and rice. In general, with the exceptions of potatoes and oilseed rape, the effects of climate change parameters on diseases of other food crops such as roots and tubers, sorghum and millets, legumes, oilseed crops as well as animal forage crops, have been neglected. Many of these crops such as cassava, sweet potato, yams, sorghum, pearl millet, Phaseolus beans and cowpea are important staples for millions of poor people in less-developed tropical countries. They are likely to face the negative effects of climate change earlier than do people in temperate areas. Clearly more research is urgently needed on these crops and their diseases.

Generally, the most important factors responsible for the outbreaks of crop plant diseases and spread into new areas are:

1) global plant trade

2) inadequate surveillance and quarantine,

3) lack of resistance in the host plant,

4) changes in virulence of the pathogen,

5) changes in crop management methods and

6) conducive weather conditions.

Prevailing weather conditions are far more important than changes in climate in the short and medium term. In a recent study on a coffee rust outbreak in Colombia attributed to climate change8, a re-examination of the dataset found no evidence for an overall trend in disease risk in coffee growing areas of Colombia from 1990-2015 and the study thus rejected the claim that the rust outbreak was due to climate change.

Care must therefore be taken to avoid jumping to the conclusion that severe and abnormal disease outbreaks are a result of climate change.

During the past 50 years, the progressive role of agricultural innovation through crop breeding and improved integrated crop management has successfully increased food production to meet our growing population needs in spite of the on-going threats from plant diseases.

As the quality of data for models is improved, crop and disease models become more integrated9 and as climate patterns become clearer over time, the predicted impacts of climate change on crop plant diseases should become more accurate.

It is therefore likely that well-focused research during the next 50 years will adapt existing management strategies and develop novel strategies to address the impacts of climate change on crop plant diseases to mitigate the threats to future food production.